Olefins, particularly ethylene and propylene, are important chemical feedstocks. Typically they are found in nature or are produced as primary products or byproducts in mixtures that contain saturated hydrocarbons and other components. Before the raw olefins can be used, they usually must be purified from these mixtures. Numerous difficulties have been experienced in this type of separation. Due to their similar relative volatilities, energy-intensive, capital-intensive, multi-trayed distillation columns typically have been used for the purification of light olefins.
An example of a prior art distillation column for the separation of propylene and propane is illustrated in FIG. 1. In an exemplary method of operation, a raw feedstock of Refinery Grade Propylene (RGP) comprising 70% propylene and 30% propane is introduced to distillation column 205 along feed pipe or pipes 200. Distillation column 205 generally comprises multiple trays, or levels. In an embodiment, distillation column 205 comprises 135 trays. Operation of a distillation column is primarily determined by a combination of the number of trays and the reflux ratio. In general, the more trays in a distillation column, the greater the separation at a constant reflux ratio, but also the greater the capital cost. Conversely, fewer trays can be used if reflux is increased, but operating cost is increased. In the distillation column, the lighter components tend to rise and the heavier components tend to sink. Accordingly, a propylene product stream is capable of being extracted along piping 235. In an embodiment, the propylene is extracted as Commercial Grade Propylene (CGP nominally 93% propylene).
A second stream comprising propylene and other lighter components can be extracted from column 205, re-condensed and injected back into column 205 along piping 210. A propane product typically greater than 95% propane, can be extracted about piping 215. The extracted products can be passed through heat exchangers 245 and/or 225 to aid in the control or change of various streams within the process.
Column 205 is merely one example of a propylene and propane distillation column. One of ordinary skill in the art would readily understand that many variations are possible.
Typical results from a single distillation column, with a 70% propylene feed, produces a propylene product of 93% purity and a propane product of 95%. Accordingly, an improved system would produce results at least as good as a typical prior art distillation column.
The art field has desired to use a membrane-based system. U.S. Pat. No. 3,758,603 and U.S. Pat. No. 3,864,418 in the names of Robert D. Hughes and Edward F. Steigelmann describe membranes used in conjunction with metal complexing techniques to facilitate the separation of ethylene from ethane and methane. Similar metal complex and membrane hybrid processes, called facilitated transport membranes, have been described in U.S. Pat. No. 4,060,566 in the name of Robert L. Yahnke and in U.S. Pat. No. 4,614,524 in the name of Menahem A. Kraus.
Further membranes have been considered for the separation of olefins from paraffins as an alternative to distillation. However, the separation is difficult largely because of the similar molecular sizes and condensability of the components desired to be separated. The membrane must operate in a hydrocarbon environment under conditions of high pressure and temperature, often resulting in plasticization and can cause loss of selectivity and/or permeation rate. Such harsh conditions tend to adversely affect the durability and stability of separation performance of many membrane materials. A membrane system with sufficiently high olefin/paraffin selectivity, high productivity, and sufficient durability in long-term contact with hydrocarbon streams under high pressure and temperature is highly desired.
The art is replete with processes said to fabricate membranes possessing both high selectivity and high fluxes. Without sufficiently high fluxes the required membrane areas required would be so large as to make the technique uneconomical. It is now well known that numerous polymers are much more permeable to polar gases (examples include H2O, CO2, H2S, and SO2) than to nonpolar gases (N2, O2, and CH4), and that gases of small molecular size (He, H2) permeate more readily through polymers than large molecules (CH4, C2H6).
However, even considering these difficulties, utilization of membrane separation has taken an important place in chemical technology for use in a broad range application. Gas separation has become a major industrial application of membrane technology in the last 15 years. Membrane based technology for the production of nitrogen from air, removal of carbon dioxide from natural gas, and purification of hydrogen now occupy significant shares of the markets for these processes.
Membrane materials and systems for separating olefinic hydrocarbons from a mixture of olefinic and saturated hydrocarbons have been reported, but none can be easily or economically fabricated into membranes that offer the unique combination of high selectivity and durability under industrial process conditions to provide economic viability.
For example, several inorganic and polymer/inorganic membrane materials with good propylene/propane selectivity have been studied. However, the art filed has found it difficult to fabricate these membranes into practical industrial membranes. Likewise, liquid facilitated-transport membranes have been demonstrated to have attractive separation performance in the lab, but have been difficult to scale up, and have exhibited declining performance in environments typical of an industrial propylene/propane stream.
Solid polymer-electrolyte facilitated-transport membranes have shown to be capable of fabrication into more stable thin film membranes for ethylene/ethane separation. See Ingo Pinnau and L. G. Toy, Solid polymer electrolyte composite membranes for olefin/paraffin separation, J. Membrane Science, 184 (2001) 39-48. However, these membranes are severely limited by their chemical stability in the olefin/paraffin industrial environment.
Carbon hollow-fiber membranes have shown promise in laboratory tests (“Propylene/Propane Separation”, Product Information from Carbon Membranes, Ltd., Israel), but are vulnerable to degradation caused by condensable organics or water present in industrial streams. Moreover, carbon membranes are brittle and difficult to form into membrane modules of commercial relevance.
Membranes based on rubbery polymers typically have olefin/paraffin selectivity too low for an economically useful separation. For example, Tanaka et al. report that the single-gas propylene/propane selectivity is only 1.7 for a polybutadiene membrane at 50° C. (K. Tanaka, A. Taguchi, Jianquiang Hao, H. Kita, K. Okamoto, J. Membrane Science 121 (1996) 197-207) and Ito reports a propylene/propane selectivity only slightly over 1.0 in silicone rubber at 40° C. (Akira Ito and Sun-Tak Hwang, J. Applied Polymer Science, 38 (1989) 483-490).
Membranes based on glassy polymers have the potential for providing usefully high olefin/paraffin selectivity because of the preferential diffusivity of the olefin, which has smaller molecular size than the paraffin.
Membrane films of poly(2,6-dimethyl-1,4-phenylene oxide) exhibited pure gas propylene/propane selectivity of 9.1 (Ito and Hwang, Ibid.) Higher selectivity has been reported by Ilinitch et al. (J. Membrane Science 98 (1995) 287-290, J. Membrane Science 82 (1993) 149-155, and J. Membrane Science 66 (1992) 1-8). However, the membrane exhibited plasticization, most likely due to the presence of hydrocarbons.
Polyimide membranes have been studied extensively for the separation of gases. Lee et al. (Kwang-Rae Lee and Sun-Tak Hwang, Separation of propylene and propane by polyimide hollow-fiber membrane module, J. Membrane Science 73 (1992) 37-45). The article discloses a hollow fiber membrane of a polyimide that exhibits a mixed-gas propylene/propane selectivity in the range of 5-8 with low feed pressure (2-4 bar).
Krol et al. (J. J. Krol, M. Boerrigter, G. H. Koops, Polyimide hollow fiber gas separation membranes: preparation and the suppression of plasticization in propane/propylene environments, J. Membrane Science. 184 (2001) 275-286) report a hollow fiber membrane of a polyimide composed of biphenyltetracarboxylic dianhydride and diaminophenylindane which exhibited a pure-gas propylene/propane selectivity of 12. However, this membrane was plasticized at even low pressures.
Many of these prior art membrane-based techniques of propane/propylene separation utilize a pervaporation mode (liquid feed, gaseous permeate) or gas separation mode (gaseous feed, gaseous permeate). In the pervaporation mode of operation, the liquid feed is “evaporated” to the permeate stream. The latent heat of vaporization is large and a large temperature decrease accompanies the separation. Pervaporation based separation has similarities to propane refrigeration. The high pressure feed permeates the membrane to low pressure. This process is similar to the expansion of propane across an expansion valve in s propylene refrigeration system.
A convenient mathematical method of describing pervaporation is to divide the separation into two steps. The first is evaporation of the feed liquid to form a hypothetical saturated vapor phase on the feed side of the membrane. The second is permeation of this vapor through the membrane to the low pressure permeate side of the membrane. Although no evaporation actually takes place on the feed side of the membrane during pervaporation, this approach is mathematically simple and is thermodynamically equivalent to the physical process.
In pervaporation, transmembrane permeation is typically induced by maintaining the pressure on the permeate side lower than the vapor pressure of the feed liquid. The permeate side pressure can be reduced, for example, by drawing a vacuum on the permeate side of the membrane, by sweeping the permeate side to continuously remove permeating vapor, or by cooling the permeate vapor stream to induce condensation. The feed may also be heated to raise the vapor pressure on the feed side or to at least partially compensate for the temperature drop on permeation.
Certain issues accompany the use of pervaporation, namely, and typically regarded as most important, the temperature decrease causes membrane productivity to decrease and further increase membrane selectivity. This is an inherent problem for pervaporation. It is very difficult to supply sufficient heat at the correct location (face of the membrane) to maintain constant temperature. As a result, productivity decreases exponentially and module count becomes unacceptably large. Further, the large temperature drop of the feed to the membrane due to the pressure drop often condenses the feed.
At least one prior art patent has identified that temperature has an effect on membrane performance. U.S. Pat. No. 5,679,133 discloses a glassy polymer membrane that is operated at temperatures of less than about 5° C. for separation of gas components. The patent claims, in an embodiment, a permeation method comprising contacting a first side of a gas separation membrane comprising a discriminating layer or region of a glassy polymer, wherein for the polymer cellulose triacetate is excluded, with the gas mixture while maintaining a difference in chemical potential from the first side of the membrane to a second side of the membrane, such that at least one first component gas of the gas mixture selectively permeates relative to at least one second component gas in the gas mixture from the first side of the membrane through the membrane to the second side of the membrane, the contacting occurring at a temperature of −5° C. or lower, wherein the membrane having a glassy region is selected so that, when using a mixture of 80 mole percent nitrogen and 20 mole percent oxygen as a feed at 30° C. with a pressure of 30 psia on the first side of the membrane and a vacuum of less than 1 mm Hg on the second side of the membrane, the permeability of oxygen in barriers is less than 2000. This patent illustrated that temperatures below ambient could be utilized in permeation procedures.
US Patent Application 2004/0000513A1 discloses a plurality of membrane modules disposed in a first product group, a second product group, and optionally one or more intermediate groups used for simultaneous recovery of a very pure permeate product and a desired non-permeate product from a mixture containing organic compounds. Examples of propylene/propane separation are given as simulated by a computer model. The preferred embodiment is a system of three membranes with propylene selectivity over propane. The embodiments disclosed pump feed stock (about 70% propylene/30% propane) in liquid form to a vaporizer and then to a first membrane. The permeate flows through, is compressed, cooled to 200° C. and passed through a second membrane. The permeate is collected as a stream comprising greater than 95% propylene. The non-permeate from the first membrane is passed to a third membrane. Permeate from the third membrane is compressed and passed back through the second membrane. Non-permeate from the third membrane is collected as a propane product.
U.S. Pat. No. 6,986,802 discloses a membrane device comprising multiple perm-selective membranes that are capable of effecting separation of a mixture of two or more compounds in a feed stock which when subjected to appropriately altered conditions of temperature and/or pressure exhibit a bubble point. The enthalpy of the feed stock is adjusted by a heat exchanger. Membrane Efficiency Index of the non-permeate fluid, when withdrawn, is within a range from about 0.5 to about 1.5. The Membrane Efficiency Index is defined as a ratio of the difference between the specific enthalpy of the feed stream entering the membrane device and specific enthalpy of the non-permeate fluid effluent to the difference between the specific enthalpy of the feed stream and the bubble point specific enthalpy of the non-permeate fluid at the non-permeate product pressure and composition. At an MEI of one, the non-permeate is disclosed as being a liquid at its bubblepoint.
U.S. Pat. No. 7,070,694 discloses an apparatus comprising a fractional distillation column and one or more membrane devices utilizing solid perm-selective membranes. The processes are stated as capable of use in simultaneous recovery of a very pure permeate product, a desired non-permeate stream, and one or more distillate products from a fluid mixture containing at least two compounds of different boiling point temperatures. The patent discloses the cooling effect produced by a membrane when a low pressure permeate is produced from a high pressure feed stock This is due to the Joule-Thompson effect. The patent further expresses the need for the incorporation heat integrated membrane apparatuses with pressure driven membrane separations.
However, these various prior art approaches require excessive capital costs and a multitude of membrane modules. Accordingly, there is a need in the art field for an improved membrane and/or membrane system for the separation of propylene and propane that uses fewer membrane modules and requires less capital cost.